Nickel Hydroxide electrode material with improved microstructure and method for making the same

Abstract

Nickel hydroxide material comprising particles with a microstructure including crystallites of less than 103 Å as measured in the 101 direction by x-ray diffraction.A method of making positive electrode material for use in an alkaline rechargeable electrochemical cell comprising forming a metal ion solution and increasing the pH to precipitate positive electrode material with a crystallite structure of 120 Å or less in the 101 direction.

Description

REFERENCE TO RELATED APPLICATIONS[0001]The present invention relates to, and is entitled to the benefit of the earlier filing date and priority of: co-pending U.S. patent application Ser. No. 09 / 686,567, entitled “A Nickel Hydroxide Positive Electrode Material Exhibiting Improved Conductivity and Engineered Activation Energy”, by Fierro et al., filed Oct. 11, 2000, which is a divisional of U.S. patent application Ser. No. 09 / 135,477, entitled “A Nickel Hydroxide Positive Electrode Material Exhibiting Improved Conductivity and Engineered Activation Energy”, by Fierro et al., filed Aug. 17, 1998, now U.S. Pat. No. 6,228,535; and to co-pending U.S. patent application Ser. No. 09 / 661,000, entitled Method of Making Nickel Hydroxide Material, filed Sep. 13, 2000, the disclosures of which are hereby incorporated herein by reference.FIELD OF THE INVENTION[0002]The present invention pertains to positive electrode materials. More particularly, the present invention pertains to nickel hydroxid...

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Benefits of technology

[0024]The present invention provides a nickel hydroxide positive electrode material having one or more of improved capacity, cycle life, rate capability, utilization and / or high temperature performance.

Problems solved by technology

However, it was conventionally recognized dogma that such gamma phase nickel hydroxide formation destroyed reversible structural stability and therefore cycle life was unacceptably degraded.

The patent states:Further, since the current density increased in accordance with the reduction of the specific surface area, a large amount of higher oxide γ-NiOOH may be produced, which may cause fatal phenomena such as stepped discharge characteristics and / or swelling.

1. Improved speed of activation, resistance to poisons, and marginal capacity improvement via increased utilization. At the present time, most commercial nickel metal hydride batteries achieve these effects through the addition of up to 5 wt % cobalt. A noted researcher, Delmas, in the Proceeding of the Symposium on Nickel Hydroxide Electrode 118-133 (1991) observed that much higher capacity could result if as much as 20% trivalent cobalt was used. However, even setting environmental and cost considerations aside, the addition of 20% Co is unstable and thus not applicable to commercially viable systems. Frequently, powdered carbon, powdered cobalt metal, and powdered nickel metal are externally also added to create separate conductive networks and thereby improve utilization. Of course, a major drawback of increasing the amount of such elements that are added is that the amount of active nickel hydroxide electrode material is correspondingly reduced, thereby reducing capacity of the electrode. Further, since Co is very expensive, the addition of Co increases cost.

2. Cycle life is extended by decreasing swelling that is initiated by density changes between the oxidized and reduced states of the nickel hydroxide material. Swelling, in turn, is accelerated by the uncontrolled density changes between βII-βIII phase nickel hydroxide and α-γ or βII-γ phase nickel hydroxide. Cd and Zn incorporated into the nickel hydroxide effectively reduce the swelling by reducing the difference in density in the charged and discharged material and increasing the mechanical stability of the nickel hydroxide material itself. This is accomplished by promoting oxygen evolution and thereby reducing charge acceptance which prevents the nickel hydroxide material from attaining the highly oxidized state (the γ-phase state). However, by suppressing or at least significantly inhibiting γ-phase state formation, the nickel hydroxide is limited to low utilization. Further, in order to effectively inhibit γ-phase nickel hydroxide, it is necessary to employ a relatively high wt % of the inhibitor element such as Zn, which high percentage results in a greatly reduced amount of active material being present thereby resulting in reduced electrochemical capacity.

3. The aforementioned “safety release” mechanism of oxygen evolution to avoid highly oxidized states (γ-phase) of nickel hydroxide material actually is an impediment to high temperature operation because a significant increase in the rate of oxygen evolution occurs with increasing temperature. The effect of such increased oxygen evolution is a very substantial decrease in utilization and ultimately a reduction in energy storage at higher temperatures in the NiMH battery using these materials. At 55° C., for example, run times of a battery may be reduced by 35-55% compared to the room temperature performance of that same battery. Elevated operational temperature conditions aside, none of these modifications of the active positive electrode material suggested by the prior art result in more than an incremental improvement in performance and none result in a significant increase in the capacity and / or utilization of the nickel hydroxide material itself, even at room temperature. All prior art batteries are limited to less than one electron transfer per nickel atom. Further, these modifications fail to address the special operational requirements of NiMH batteries, particularly when NiMH batteries are used in electric vehicles, hybrid vehicles, scooters and other high capacity, high drain rate applications. Because NiMH negative electrodes have been improved and now exhibit an extremely high storage capacity, the nickel hydroxide positive electrode material is essentially the limiting factor in overall battery capacity. This makes improving the overall electrochemical performance of the nickel hydroxide material in all areas more important than in the past. Unfortunately, the elements currently added or previously suggested to be added to the nickel hydroxide material result in insufficient improvements in performance before competing deleterious mechanisms and effects occur. For example, Cd cannot be used in any commercial battery because of the environmental impact thereof, and Co and Zn appear to become most effective only at levels that result in a significant decrease in cell capacity; more specifically, energy per electrode weight.

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